1. Field of the Invention
The present invention relates to a vacuum interrupter used with an electric circuit of high power, for example, an alternating current circuit of high power. More particularly, the invention pertains to a vacuum interrupter including means for applying a magnetic field to an arc in parallel to a longitudinal axis of the arc (hereinafter, the magnetic field is referred to as an axial magnetic field) which is established across a space between a pair of contact-electrodes within a vacuum envelope of the vacuum interrupter when the contact-electrodes are into in or out of engagement, thus enhancing current interruption capability of the vacuum interrupter.
2. Description of the Prior Art
Recently, it has been required to provide a vacuum interrupter of much enhanced large current interrupting capability and dielectric strength to cope with increasing current and voltage of power lines with an expansion of an electric power supply network.
A vacuum interrupter employing an axial magnetic field, which includes a pair of contact-electrodes, restricts the electric arc to a space between the contact-electrodes with the applied axial magnetic field uniformly diffusing the arc in the space, when the contact-electrodes are separated, thus preventing any concentrating arc-spot of the contact-electrodes from locally overheating and thus enhancing the current interruption capability and dielectric strength thereof.
Generally, the contact-electrode itself is required to consistently satisfy the following requirements:
(i) low electrical resistivity, PA1 (ii) high current interruption capability, PA1 (iii) high dielectric strength, PA1 (iv) high anti-welding capability, PA1 (v) high leading and lagging small current interruption capabilities, PA1 (vi) low amount of chopping current, and PA1 (vii) low erosion.
However, a contact-electrode to consistently satisfy all the above requirements, in the present state of the art, has not been provided.
For example, a disc-shaped contact-electrode of copper which includes a plurality of radial slits is presented as a contact-electrode of a well-known vacuum interrupter of an axial magnetic field applying type. The disc-shaped and slitted contact-electrode has certain advantages in that it reduces eddy currents so as not to weaken the axial magnetic field. However, the small tensile strength of copper, which amounts to 20 kgf/mm.sup.2 (196.1 MPa), and the plurality of slits cause mechanical strength of the disc-shaped and slitted contact-electrode to be much reduced. Thus, the thickness and weight of the contact-electrode must be increased in order to prevent a deformation of the contact-electrode due to the mechanical impact and electromagnetic force from large currents which are applied to the contact-electrode when the vacuum interrupter is closed and opened.
In addition, electric fields and multiple arcs are concentrated at edge portions of the slits which reduces the dielectric strength between the contact-electrodes, particularly the dielectric strength after an interruption of a large current (hereinafter, referred to as dynamic dielectric strength) and erodes the contact-electrode (refer to U.S. Pat. No. 3,946,179).
In addition, there are known as examples of a pair of contact-electrodes of a vacuum interrupter of an arc driving type but not as those of a pair of contact-electrodes of the vacuum interrupter of the axial magnetic field applying type, various contact-electrodes, which are adapted for large currents of low voltage. These contact-electrodes are made of copper alloyed with a minor constituent of a low melting point and a high vapor-pressure, such as a contact-electrode of copper alloyed with 0.5% bismuth by weight (hereinafter, referred to as a Cu-0.5Bi alloy) which is disclosed in the U.S. Pat. No. 3,246,979, or a contact electrode which is disclosed in the U.S. Pat. No. 3,596,027.
Such contact-electrodes of copper alloyed with a minor constituent of a low melting point and high vapor-pressure as a contact-electrode of Cu-0.5Bi alloy are excellent in large current interrupting capability, electrical conductivity and anti-welding capability, whereas significantly low in dielectric strength, particularly in dynamic dielectric strength. In particular, a current chopping value of a pair of contact-electrodes of Cu-0.5Bi alloy amounts to 10A, being relatively high, so that it happens to cause harmful chopping surges in the current interruption. Thus, a pair of contact-electrodes of Cu-0.5Bi alloy are not well suited in lagging small current interrupting capability, which happens to lead to dielectric breakdown of electrical devices of inductive load circuits.
For overcoming the drawbacks of the contact-electrode of copper alloyed with a minor constituent of a low melting point and a high vapor-pressure, there are known various contact-electrode of alloy consisting of copper and a material of a high melting point and a low vapor-pressure, such as a contact-electrodes of alloys consisting of 20% copper by weight and 80% tungsten by weight (hereinafter, referred to as a 20Cu-80W alloy) which is disclosed in the U.S. Pat. No. 3,811,939, or a contact-electrode which is disclosed in the GB-No. 2,024,257A.
Such contact-electrode of alloys consisting of copper and a material of a high melting point and a low vapor-pressure as a contact-electrode of 20Cu-80W alloy above is relatively high in static dielectric strength, whereas relatively low in large current interrupting capability.